submachine gun
A compact automatic firearm that fires pistol-caliber ammunition, used by military and police forces.
The special forces are equipped with submachine guns.
機関銃 is a general term for machine gun, often larger and using rifle-caliber ammunition, while 短機関銃 specifically refers to a smaller submachine gun.
Compound of 短 (short) + 機関銃 (machine gun), literally 'short machine gun'. The term is a direct calque of the English 'submachine gun'.